In an agricultural harvesting-threshing vehicle such as a combine, the harvested crop is conventionally acted upon by a series of crop-processing mechanisms such as a threshing cylinder and cooperating concave, straw walkers, a shaker shoe, etc., and the combine operates most efficiently when such crop-processing mechanisms are continuously supplied with a predetermined load of crop material. If too much crop material is being supplied, such crop-processing mechanisms become overloaded. If too little crop material is supplied, the combine is not working up to capacity. Overloading of the threshing cylinder, the straw walkers and the shaker shoe results in higher loss of grain.
Combines are often provided with an infinitely variable transmission and manual means for selectively controlling the transmission ratio, and thus combine ground speed, so that the operator may adjust the amount of crop material supplied to the crop-processing mechanisms in accordance with crop conditions, i.e., to permit reducing ground speed, and thus reduce the amount of crop material supplied, when moving into a region of dense crop and increasing ground speed, and thus increase the amount of crop material supplied, when the crop is relatively thin.
Combine speed controls are also known for automatically adjusting combine ground speed. In the control of U.S. Pat. No. 3,073,099 to Anderson, the drive train between the engine and the ground wheels includes a variable diameter pulley, and the transmission ratio is controlled in response to the output of a device which senses the volume rate of flow of crop material into the combine. In U.S. Pat. No. 3,442,068 to Bulin, the instantaneous crop load is automatically monitored by sensing variations in the separation between a threshing cylinder and its associated concave and translating such variations into opening and closing of a shunt valve which bleeds pressure from a hydraulic device that controls combine ground speed. The combine ground speed control of U.S. Pat. No. 3,481,122 to Pool et al senses both the torque required to feed the crop material into the harvester and the torque required to process the crop material and regulates combine ground speed in response to the algebraic sum of such measured torques. U.S. Pat. No. 3,514,929 to Cornish et al discloses an electronic control system for a combine in which a torque sensing transducer in the drive shaft for the grain harvesting header generates an electrical feed rate signal proportional to the torque in the header drive, and such feed rate signal is combined with a feed rate reference signal that is compared, in turn, with a velocity setpoint signal and a velocity signal from a transmission transducer, the sum of said signals being used to control an infinitely variable transmission.
In the automatic speed control for a combine disclosed in U.S. Pat. No. 4,130,980 to Fardal et al, the torques applied to the threshing cylinder drive shaft and to the crop feeding drive shaft are sensed by idler pulleys which engage the tight side of drive belts for such shafts and actuate potentiometers that generate electrical signals which control a hydrostatic drive system for the combine drive wheels so that ground speed is regulated in response to measurement of the power required to drive the threshing cylinder and also the power required to drive the crop cutting and conveying mechanisms.
One disadvantage of certain of such prior art automatic combine speed control systems is that they respond to only a single variable and can result in sharp changes in ground speed since the torque required (or load measured) at one point is neither anticipated nor remembered. Another fault of such speed control systems based on grain loss input functions is that their response time is slower than that of the operator due to the material transport time required to move the crop through the combine. A time period of up to twenty-five seconds may be required to transport the crop through the combine, and in twenty-five seconds the combine may have encountered crop conditions where the ground speed is sufficiently high to push the crop over and stall the engine. The combine operator can react quicker than such transport time and manually adjust combine ground speed to compensate for such change of crop conditions before the automatic control responds. Further, in prior art automatic combine speed controls wherein load or torque is monitored by an idler pulley on the tight side of a drive belt, calibration is very sensitive and necessitates that the power sensing mechanism be zeroed frequently in the field.